Snag characteristics and cavity-nesting birds in the unmanaged post-fire northeastern Canadian boreal forest

Abstract

We studied the availability and characteristics of snags and their use by cavity-nesting birds in the northeastern part of the Canadian boreal forest. We built up two long-term (> 200 years) chronosequences following time since the last fire in the unmanaged boreal forest of northeastern Québec, one in the balsam fir-white birch domain (southern region) and one in the spruce-mosses domain (northern region). We then sampled and characterized snags and live trees in 30 stands from each of these two chronosequences. We also looked for nest cavities on all sampled snags, performed bird inventories by point counts, and calculated tree mortality rate from permanent sample plots. Results show that mortality rates follow a U-shaped pattern, with more snags of large diameter (> 20 cm DBH) in young (< 50 years) and in old (> 200 years) forests. In the latter, we also found more nest cavities than in any other age classes. Although abundance of primary cavity nesters (excavating species) did not vary among age classes, secondary cavity nesters (using cavities already available) tend to be more numerous in older forests. Our results highlight the capacity for young and old-growth forests to provide quality habitat for species that are dependent on large snags. Proper forest management should maintain a mosaic of different age forest stands, including snags, to promote biodiversity and provide important resources for resident bird species.Keywords snags, cavity nesting birds, boreal forests, old-growth, long term chronosequence, time since fire, mortality rate, basal area Addresses Lowe and Pothier, Centre d'étude de la forêt, Département des sciences du bois et de la forêt, Pavillon Abitibi-Price,

Temporal changes in stem decay and dead and sound wood volumes in the northeastern Canadian boreal forest

“…Another predictive difficulty was related to the variable proportion of DSWH3 in the overall dead wood pool that was present in a forest stand. Instead of presenting a typical U-shaped pattern over time when all degradation classes are taken into account (Table 3b; Harmon et al 1986;Lowe et al 2011), the pattern that we observed for the volume of DSWH3 differed, albeit slightly (Fig. 4), and was more prone to rapid changes.…”

“…On the other hand, the availability of dead standing trees (snags) through forest developmental stages has been studied across different parts of the circumboreal forest and generally has been described as a "U-shaped" temporal pattern: the occurrence of snags is generally high immediately after a stand-replacing disturbance such as fire and then decreases in regenerating stands to increase again as the stand matures (Brassard and Chen 2006;Harmon et al 1986;Harper et al 2005;Kankrina and Harmon 1995;Lowe et al 2011, Nilsson et al 2002. Dead standing trees continually deteriorate and eventually fall over (Harmon et al 1986) after a certain period of time, depending on species and environmental conditions (Aakala et al 2008;Angers et al 2010).…”

“…The northeastern Canadian boreal forest contains a relatively high proportion of dead standing trees compared with many other boreal ecosystems (Lowe et al 2011). This is mainly attributable to the high proportion of old-growth forests (Boucher et al 2003;Côté et al 2010), which is a consequence of the long fire cycle (>300 years) that often exceeds the life span of individual trees (ϳ200 years) (Bouchard et al 2008;Foster 1983).…”

“…These old-growth forests are thus generally composed of numerous balsam fir trees, which could maintain or increase tree mortality rates (Aakala et al 2008;Boucher et al 2006;Lowe et al 2011), as the species is generally shorter lived and more vulnerable to spruce budworm than black spruce (MacKinnon and MacLean 2004;Burns and Honkala 1990).…”

The NEBIE plot network: Highlights of long-term scientific studies

“…Species diversity may be associated with the presence and abundance of structural features as well as with the variability of these features in terms of their size, growth stage, and state of decay. To understand the effects of forest practices on species diversity, it is important to study potential drivers of this diversity such as the abundance and variability of coarse woody material, including standing dead stems (snags) (Drapeau et al 2009, Lowe et al 2011) and downed wood (Crites andDale 1998, Dechene andBuddle 2010), as well as tree size diversity, spatial variability in microtopography (e.g., pitsand-mounds), and forest floor characteristics (Spies 1998).…”